(1) Field of the Invention
The invention relates to a current reference circuit, and more particularly, to a threshold voltage-independent MOS current reference circuit.
(2) Description of the Prior Art
Current and voltage reference circuits are widely used in analog designs. A particularly difficult problem encountered in MOS reference circuit designs is caused by the large variation in threshold voltage (Vth) that often occurs in CMOS processing. Since the voltage-to-current transfer response of the MOS transistor depends on the value of Vth, large variations in Vth can cause large variations in the actual current or voltage output of the reference circuit. It is desirable, therefore to eliminate Vth dependence in the reference output.
However, prior art attempts to eliminate the Vth component typically rely on complicated voltage addition techniques to create a Vx+Vth. These techniques create several problems due to the use of differing operation points, or modes, for different MOS devices. Therefore, mismatch problems are a major drawback.
Several prior art inventions describe voltage or current reference circuits. U.S. Pat. No. 5,739,682 to Kay describes a reference substantially independent of the threshold voltage of the transistor providing the reference. A pair of MOS transistors has gate voltages made equal. The current through the first transistor is very small. The current through the second transistor is equal to the first current multiplied by a scaling factor. Since the first current is so small, the second current through the second transistor is essentially not dependent upon the threshold voltage. U.S. Pat. No. 5,910,749 to Kimura teaches a current reference with no temperature dependence. Both bipolar and MOS embodiments are disclosed. U.S. Pat. No. 4,723,108 to Murphy et al describes a circuit to compensate for MOS transistor performance changing over temperature and manufacturing variation. Changing Vth, caused by temperature, is compensated by changing the mobility in the opposite direction. The gate drive of a MOS device is thereby compensated. U.S. Pat. No. 5,315,230 to Cordoba et al teaches a reference voltage generator circuit that compensates for temperature and VCC variation.
A principal object of the present invention is to provide an effective and very manufacturable current reference circuit.
A further object of the present invention is to provide a current reference circuit comprising MOS devices.
A still further object of the present invention is to provide an MOS current reference circuit that is independent of the threshold voltage to thereby reduce reference current variation due to processing variation.
Another still further object of the present invention is to provide a nearly zero temperature coefficient current references using this novel MOS current reference circuit.
In accordance with the objects of this invention, a new current reference circuit is achieved. This current reference circuit uses MOS transistors. However, the reference value does not depend upon the threshold voltage. The circuit comprises, first, a first MOS transistor having gate, drain, and source. A gate voltage value is coupled from the gate to the source. A second MOS transistor has gate, drain, and source. The second MOS transistor is of the same size and type as the first MOS transistor. The source is coupled to the first MOS transistor source. The gate voltage value plus a delta voltage value is coupled from the gate to the source. A means is provided for forcing a drain voltage value from the drain to the source of the first MOS transistor and from the drain to the source of the second MOS transistor. The first MOS transistor and the second MOS transistor conduct drain currents in the linear mode. Finally, a means is provided for subtracting the first MOS transistor drain current from the second MOS transistor drain current to thereby create a current reference value. The current reference value does not depend upon the threshold voltage of the first and second MOS transistors. The circuit may be further applied to create a nearly zero temperature coefficient current reference.